This invention relates generally to power bridge circuits, and, more particularly, to non-dissipative snubber circuits suitable for use in such bridge circuits.
Power bridge circuits are commonly used as power inverters for converting dc power into ac power. In such inverters, two switching power transistors are connected to the primary winding of a power transformer and arranged such that one couples electrical current through the winding in a first direction and the other couples electrical current through the winding in a second, opposite direction. Driver means controllably switches on and off the two transistors in a prescribed sequence and at prescribed duty cycles, to produce a pulse sequence at the transformer's primary winding. This sequence is low-pass filtered to produce the desired ac power signal. In a half-bridge configuration, the side of the transformer's primary winding opposite the two transistors is connected directly to a fixed voltage, whereas in a full-bridge configuration, the primary winding's opposite side is connected to an additional pair of transistors, which cooperate with the first two transistors to couple current through the winding in the two opposite directions.
A snubber circuit is commonly used with each transistor in bridge circuits of this kind, to prevent the simultaneous presence of high voltage and high current in the transistor junction each time it is biased off. This condition of high voltage and high current arises because the load imposed on the transistor by the transformer's primary winding is highly inductive, and the load current therefore continues to flow, even after the transistor has been biased off. The snubber circuit serves as a bypass route for this current, avoiding the transistor junction.
Snubber circuits of this kind typically include a series diode and capacitor shunting each transistor. When the transistor is biased on, the diode ensures that the capacitor does not affect the transistor's operation. When the transistor is first biased off, however, the current that previously flowed through the transistor and that continues to flow because of the load's inductance, is diverted through the diode to charge the capacitor to a predetermined voltage. Thereafter, when the transistor is again turned on, a special circuit diacharges the capacitor to its original state. This discharge circuit frequently takes the form of a resistor connected in parallel with the diode, such that the discharge current is routed through the resistor and transistor. One important drawback to such a circuit configuration is that it can dissipate substantial amounts of energy. Moreover, this energy dissipation cannot ordinarily be reduced without increasing the snubber circuit's recovery time.
Snubber circuits that are non-dissipative of significant amounts of energy have been devised for use in some situations. These circuits typically include an inductor that resonates with the snubber capacitor, to discharge it in a relatively short time duration, with little energy dissipation. Although these prior non-dissipative snubber circuits have proven generally satisfactory for their limited purposes, they are believed to be inherently incompatible with power bridge circuits. Moreover, there is no readily apparent way that these snubber circuits can be modified to make them compatible.
It should therefore be appreciated that there is a need for a snubber circuit that dissipates very little energy, has a rapid recovery time, and is particularly suited for use in a power bridge circuit configuration. The present invention fulfills this need.